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I have several devices plugged into a powered USB hub which in turn is connected to the PC via a USB extension cable.

When I plug the devices into the USB hub and then the hub into the PC, the PC loses the connection to the devices every few seconds. Even when all USB devices are self-powered.

However, when I first plug the USB hub into the PC and then plug in all devices into the USB hub, everything works.

With a shorter USB extension cable, everything works regardless of the plugging order.

Does that mean that the powered USB hub is badly programmed (sending too weak USB data signal to the PC, depending on the plugging order), whereas the USB extension cable is probably good?

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    How long is the USB extension? - extension cables are often out of spec for USB - 5meters for USB2, less forvUSB3 - so it may come down to the quality of the extension cable.
    – davidgo
    Commented Apr 1, 2023 at 9:26
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    This article states : "Gen 2 or 20Gbps cables can be up to 2m. A cable longer than that can still function but may degrade performance." and also "Passive lower-cost cables are only capable of 20Gbps data transfer at 1m or 2m lengths but can achieve full 40Gbps at a shorter cable length of 0.5m." How long is your cable?
    – harrymc
    Commented Apr 1, 2023 at 9:37
  • @davidgo The cable is 5 meters long. But why do results depend on whether I plug in the computer first vs. the devices first?
    – root
    Commented Apr 1, 2023 at 9:56
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    Guess: This long cable cannot do fast communication so it scrambles it just enough to confuse the USB driver when multiple devices try to work simultaneously.
    – harrymc
    Commented Apr 1, 2023 at 10:01
  • @harrymc But then why does it work if I plug in the PC first and then all the devices?!
    – root
    Commented Apr 1, 2023 at 10:04

1 Answer 1

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With a shorter USB extension cable, everything works regardless of the plugging order.

You've got your answer right there: USB cable length is limited, for two reasons

  • Signal deterioration: the longer the line, the weaker the signal, and the more interference both inside the cable itself and with other equipment.
  • Response timing: Subsequent USB standards have shorter and shorter response timeframes. If the response doesn't come in that timeframe, negotiation will fail.

In the situation where all peripherals try to connect at the same time, you probably have interference inside the cable between the different signals (Cable crosstalk), which gets worse as the signal gets weaker. If you connect the peripherals one-by-one, the signal is just strong enough to arrive.

You could try with a different high-quality cable or a different hub, but for 5 m lenght no-one can guarantee this will work with standard cables, not even with active cables. Also, you might get it to work today, but it will fail tomorrow. USB-Specs 3.0 and 3.1 have 2m as maximum lenght for passive cable, and active cables usually reach 3m.

That said, you've got some different options:

(A) Optical USB cables will transform your signal at both ends so it can be sent over an optical fibre, and they can be long up to 100 m. Optical cables are very fast, so timing usually is not an issue. However, they can be very expensive.

(B) USB Signal Boosters or repeaters amplify your signal so it will travel farther. However, they are mostly meant to connect one only peripheral, so choose your product well to be sure you can connect a Hub. Also, you might experience lower speeds due to package loss for timing reasons.

(C) Daisy-chain two or three powered USB-hubs:

PC >---2-3m---< HUB 1 >---2-3m---< HUB 2 --> peripherals

This should eliminate timing problems since the HUB 1 will work as intermediary.

You could also consider to put your Hub max. 3m from your PC, and from there on run multiple long cables to each of your periphery.

(D) For completeness, there exist also USB-over-Ethernet-extenders, but they are usually limited to USB 2.0 because of too slow timing for USB 3.0 and speed limits of Ethernet cables.

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  • What exactly do you mean by "If you connect the peripherals one-by-one, the signal is just strong enough to arrive."? After I connect and use them, they send signals most of the time (why with less "interference inside the cable between the different signals"?), but don't get disconnected. So how is that phase different from what happens after I connect the computer last?
    – root
    Commented Apr 2, 2023 at 17:30
  • I don't know what you mean by "interference inside the cable between the different signals". Different signals don't get sent simultaneously because USB is serial.
    – root
    Commented Apr 2, 2023 at 17:32
  • USB 3.0 onwards is full duplex: if your PC sends a strong signal, while a weak signal arrives from the other side, the weak signal will experience interference, and won't arrive correctly. That's called interference inside the cable. See my link about crosstalk
    – 1NN
    Commented Apr 2, 2023 at 18:28
  • How does having connected the PC last a long time ago make the arriving signal weaker than having connected the PC first? (Note that everything works if I connect the PC first.)
    – root
    Commented Apr 2, 2023 at 18:32
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    There's no easy way to know exactly what is happening. But I can imagine your USB connection through that cable is slow because of package loss due to interference. If a package does not arrive, the device will re-send it, until the other part confirms it has arrived correctly. However, the initial handshake between devices to establish connection is different, and probably is more problematic
    – 1NN
    Commented Apr 2, 2023 at 18:36

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